1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy and, more particularly, to a glass-to-metal seal (GTMS) for hermetically sealing an electrochemical cell. The glass-to-metal seal is considered critical because it hermetically isolates the internal environment of a component from the external environment to which the component is exposed. In electrochemical cells powering implantable medical devices, the GTMS hermetically seals the internal cell chemistry from the external device environment.
2. Prior Art
The glass-to-metal seal of an electrochemical cell typically consists of a ferrule sleeve secured to an opening in the cell casing, such as in the lid or in the casing body itself. The ferrule supports an insulating glass in a surrounding relationship and the glass in turn seals around the perimeter of a terminal lead. The terminal lead extends from inside the cell to a position outside the casing, and serves as the lead for one of the cell electrodes. Typically the terminal lead is connected to the cathode current collector. The casing including the lid serves as the second terminal for the other electrode, typically the anode. This configuration is referred to as a case-negative design.
To construct a glass-to-metal seal, insulating glass is generally provided in a ring or tubular shape to fit inside the ferrule sleeve or inside an opening in the casing body. The insulating glass has a hole through its center that receives the terminal lead. These components are assembled in a closely spaced relationship and then heat treated in a furnace. This heating step causes the glass to soften and flow into intimate contact with the inside of the ferrule and with the perimeter of the terminal lead. When the assembly cools, the insulating glass is bonded to the ferrule and the terminal lead creating a glass to metal seal therebetween.
In some electrochemical cell constructions, two or more terminal leads with respective glass-to-metal seals are utilized. Each of these terminal leads may be electrically connected to the current collector of the cathode, the anode or they may be connected to the casing of the cell to facilitate proper operation.
There have historically been various methods in the construction of these multiple terminal lead electrochemical cells. One method forms multiple glass-to-metal seals of the terminal leads simultaneously. More specifically, the insulating glass is positioned about the perimeters of the first, second, or more of the terminal leads. The glasses and leads are placed inside the ferrule sleeve or inside respective openings in the casing body in a closely spaced relationship. The assembly of leads is then heated to form the multiple GTMS at one time. This method of simultaneously forming the plurality of GTMS is not ideal because it requires complex tooling. Each terminal lead must be located independently to account for expansion of the header and tooling at the glassing temperature. These components ensure the proper positioning of the multiple leads within the header during the heating process. Furthermore, if the header is too small, there may not be sufficient space for the leads and their associated lead locating components.
The second option is to form the glass to metal seals independent of each other. In this method, individual ferrules or sleeves are used to isolate each of the terminals. This second option is not ideal because the current process requires multiple positioning components and additional processing steps.
What is therefore needed is an electrochemical cell comprised of a multitude of glass-to-metal hermetically sealed terminal leads and a forming process thereof that does not require the use of special lead positioning components or processing steps. The present invention provides an electrochemical cell with multiple glass to metal seals and a manufacturing process thereof that overcomes the shortcomings of the prior art.